Recently, a Nd-Fe-B system magnet has been developed which exhibits a maximum magnet energy product superior to a Sm-Co system magnet. The aforesaid magnet is considered to be useful in various applications such as in the area of high-performance miniature magnets.
It is generally known that the maximum magnetic energy product of a magnet material composed of a rare earth element and Fe or Fe and Co is greatly improved by imparting thereto a magnetic anisotropic property. In this regard, a number of processes for producing a Nd-Fe-B system magnetic material having excellent magnetic characteristics have been proposed.
As an example of a typical process for preparing a Nd-Fe-B system magnet having magnetic anisotropy, it is proposed in JP-A-59-46008 (the term "JP-A" as used herein means an "unexamined published Japanese patent application" that the magnet can be prepared using a conventional powder metallurgy technique. JP-A-59-46008 further describes a process for preparing an anisotropic magnet comprising preparing an alloy ingot of Nd, Fe, and B, next pulverizing the alloy ingot into a fine powder, and then consolidating the powder in a magnetic filed followed by sintering.
However, the anisotropic magnet produced by the above described process is disadvantageous in that the magnet can not be used as a magnetic powder for preparing a bonded magnet.
Apart from the above described powder metallurgy method, the rapid quenching method from the melt proposed in JP-A-59-64739 and JP-A-59-211549 describes a process for preparing a magnet material by forming a ribbon-form amorphous alloy from a molten alloy of Nd, Fe, and B. A rapid quenching technique such as melt spinning is then used followed by heat-treating the amorphous alloy ribbons to crystallize the Nd.sub.2 Fe.sub.14 B phase.
Furthermore, JP-A-60-9852 proposes a quenched ribbon form magnet material containing fine crystal particles of a Nd.sub.2 Fe.sub.14 B phase.
The magnet material disclosed in JP-A-59-64739, JP-A-59-211549, and JP-A-60-9852 as noted above have a high intrinsic coercive force of from 8 kOe to 15 kOe but are disadvantageous in that the magnet materials are isotropic and the maximum magnetic energy product, an important magnetic property, is not sufficiently increased.
A process for preparing an anisotropic magnet using flakes of amorphous alloy ribbons produced by a rapid quenching technique is proposed in JP-A-60-100402. The process comprises first hot pressing powdered Nd-Fe-B system amorphous alloy ribbons, next hot-deforming the bulk and orienting the axes of the crystal that are reading magnetized in the same direction based on the plastic flow. However, the production process of the above described anisotropic Nd-Fe-B system magnetic material is disadvantageous in that complicated steps are required, the production time is inevitably prolonged, the productivity is low, and the production cost is very high.
On the other hand, a high-performance miniature magnet other than the above described quenched ribbon-form magnet material and a process of producing the same are proposed in JP-A-1-180757. Particularly, JP-A-1-180757 describes a fibrous Nd-Fe-B system hard magnetic material having a diameter of less than 500 .mu.m formed by spinning into fiber-form and solidifying. JP-A-1-180757 further describes that the magnetic material can be produced by method of spinning in a rotating liquid using water as the cooling medium.
However, when the present inventors sought to produce a Nd-Fe-B system hard magnetic material by a method of spinning in a rotating liquid using water as the cooling medium in accordance with the teachings of JP-A-1-180757, a miniature fibrous material having a diameter of less than 500 .mu.m was obtained. However, the surface of each fiber was covered by a thick oxide film, the intrinsic coercive force (iHc) (a magnetic characteristic of the resulting material was only from about 3 kOe to 5 kOe, and the maximum magnetic energy product was even less. Thus, the present inventors have determined that a fibrous permanent magnet having excellent magnetic characteristics is not obtained by the process described in JP-A-1-180757.
Also, a method of spinning in a rotating liquid using water as the cooling medium for obtaining a fibrous permanent magnet is disadvantageous in that there is almost no difference in the fibrous permanent magnet thus obtained between the magnetic characteristics (e.g., coercive force and residual magnetic flux density) in the lengthwise direction of the fiber axis and those in the direction perpendicular to the fiber axis. In other words, a fibrous permanent magnet having anisotropy is not obtained.